1. Field of the Invention
This invention relates generally to a system and method for estimating battery capacity and, more particularly, to a system and method for estimating battery capacity while the battery is in service, where the method includes dividing integrated current amp-hours from a previous drive cycle by a change in battery state-of-charge (SOC) between the previous drive cycle and a current drive cycle after the battery has rested for a predetermined period of time that is based on temperature.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV), that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium-ion, nickel metal hydride, lead-acid, etc. A typical high voltage battery system for an electric vehicle may include a large number of battery cells or modules including several battery cells to meet the vehicle power and energy requirements. The battery system can include individual battery modules where each battery module may include a certain number of battery cells, such as twelve cells. The individual battery cells may be electrically coupled in series, or a series of cells may be electrically coupled in parallel, where a number of cells in the module are connected in series and each module is electrically coupled to the other modules in parallel. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
When a battery pack is relatively new, each cell in the battery pack typically operates at about the same level of performance, i.e., maximum charge or capacity. However, as the battery pack ages over time, each cell typically degrades in performance differently than the other cells, where the performance of the battery pack is limited by the performance of the lowest performing cell. Further, a battery cell or battery module in the battery pack may fail or may otherwise be limited in performance for other reasons, such as an internal short, loss of capacity, high resistance, high temperature, etc.
The control algorithms in an electric vehicle typically need to know the capacity of the battery to track or determine battery SOC as the battery ages for battery end of life concerns. Known algorithms that estimate battery capacity have heretofore been less effective than they should be because it is typically difficult to know whether the various inputs for making the calculations to determine battery capacity are valid or proper at any particular point in time.